Mines are awesome. I don't know why you posted this in strategy but you have to hit "G" as a missle troop to select your "second secondary" weapon. This will change your nades to mines and then you just press alt-fire to lay them.

This is also the same way you use everyone else's secondary (health/ammo, recon droid for orbital strike, extra nades etc.)

The play is so frantic people NEVER have time to look for mines. I've been getting massive kills just laying mines in popular spots. They don't trigger on your enemies so just place them anywhere (particularly in high grasses, enemy spawn points, camp points, etc.)